The present invention relates to bellows-type expansion joints, and more particularly to reinforced expansion joints which are adapted to couple lengths of rigid conduit together while allowing relative movement to occur.
While various types of resilient expansion joints have been known for years, such joints generally take one of two forms. One form, which will be termed the shallow or flowing arch type, has generally evolved for use in smaller-diameter applications wherein a uniformly resilient joint may be provided. Examples of shallow-arch joints are shown in U.S. Pat. Nos. 3,359,014--Clements and 3,552,776--Leymann. Joints or couplings of this type have a gradually enlarged section intermediate their ends which acts as a bellows to enable axial contraction or elongation of the joint, and moreover to accommodate angular and transverse misalignment of the ends. The body or carcass of the joint is generally of uniform construction, that is, the cross-sectional buildup or formation of the carcass is consistent from one end to the other. Shallow-arch joints are conventionally manufactured by a molding process which effectively limits their size due to the bulk and expense of the necessary molds.
In forming the reinforcing layer of flowing-arch expansion joints it is conventional to use a fabric which has been cut on a bias so that the warp and/or weft of the fabric extends at some angle to the axis of the joint. As set forth in U.S. Pat. No. 3,359,014 the preferred angle for such bias-cut layers is generally 45.degree.. This provides both axial and radial support to the structure, yet allows the materials to be distorted sufficiently to be formed in the desired shape wherein the center of the material is distended to a larger diameter than the ends thereof, i.e. material is wrapped around a mandrel or mold core to form a bulged cylinder.
Owing to the deficiencies inherent in such a construction wherein flexure must take place over a large proportion of joint length, efforts have been made to add additional reinforcing structure. For instance, as in U.S. Pat. No. 3,359,014 a metal band may be placed about the joint at its center; or, as shown in U.S. Pat. No. 3,552,776 a reinforcing insert of metal or the like may be molded within the elastomeric body. While the shallow-arch type of expansion joint has the attributes of relatively low cost and weight in sizes up to approximately 20 inches in diameter, the relatively low strength factor and high tooling costs have prevented this design from becoming popular in large-diameter applications.
By far the most popular design of expansion joint for high pressure and/or high temperature applications is the so-called high arch design, as exemplified in U.S. Pat. Nos. 3,580,616 and 3,429,592--Merkwacz. The arch of this joint has substantially straight walls, and occupies only a relatively small, highly-defined portion of the total length of the joint. The walls of the arch are substantially perpendicular to the axis of the joint so as to form a sharp, narrow bellows in the joint. It has been found that the high arch joint lends itself to an extremely strong mode of construction, inasmuch as rigid reinforcing rings can be formed within the straight portions of the body or externally over the straight sections of the expansion joint on either side of the straight-walled arch. Although such reinforcement does not extend into the arch of the joint, due to the sizing and proportioning of the high, narrow arch a highly pressure-resistant joint can nonetheless be formed.
Although the so-called high arch expansion joint has been developed to a high degree, certain problems inhere in its construction and use. In order to achieve the required rigidity, the reinforcing rings in or over the body of the joint must be firmly and permanently located. Excess motion of the joint tends to break the internal rings free of the surrounding material, causing them to move or migrate within the carcass of the joint. Continued movement of this sort eventually results in destruction of surrounding material, and failure of the joint. As shown in U.S. Pat. No. 3,580,616 other methods of reinforcing the linear portions of the joint have been developed; generally, the approach is to surround as much of the joint as possible with an inextensible material which will prevent the joint from distending, and relegate any movement to the bellows or arch portion of the joint.
In order to overcome the inherent propensity of the high-arch joint to collect particulate material and sediment, a soft or sponge-like filler may be placed in the annular cavity formed by the arch. While this prevents the accumulation of particulate material it also inhibits movement of the joint, and commonly decreases the elongation, compression, and transverse distortion of the joint by 50%.
Attempts have been made to achieve the benefits of both types of joints in a single structure; for example, in U.S. Pat. Nos. 3,206,228 and 3,799,825 various types of supporting structures are shown which are intended to lend additional strength to expansion joints. U.S. Pat. No. 3,799,825 illustrates the prior art method of building up the carcass of an expansion joint wherein the fabric used to build up the carcass of the joint is cut on a bias such that the angle of the strands of the fabric is very high with respect to the axis of the joint. By using narrow lengths of material in which the strands run substantially crosswise, the strands are discontinuous and thus do not fully encircle the body of the joint. Since the strands only extend partway about the conduit a measure of expansion is possible through a stretching of the elastomer lying between adjacent sets of strands. As in the case of the above-described high-arch joint construction, reinforcement intermediate the arches can be provided by disposing helical metal ribbons upon the cylindrical walls of the assembly.
From the foregoing, it will be appreciated that it would be highly desirable to provide a large, high-pressure expansion joint which exhibits the strength of the high-arch expansion joint, but with the flexibility, low-weight and lack of particle entrapment which characterizes the flowing-arch style of joint.
It is therefore an object of the present invention to provide an expansion joint substantially as strong as conventional reinforced high-arch joints, without the presence of rigid rings imbedded therein or installed over the straight body sections.
Another object of the present invention is to provide an expansion joint which is substantially lighter than presently-available joints of equivalent pressure ratings.
Yet another object is to provide an expansion joint whose strength is comparable to a reinforced high-arch style of joint, but which does not entrap particulate material.
Yet another object is to provide an expansion joint of the low-arch configuration which will withstand internal pressures as well as a reinforced high-arch joint.
Yet another object is to provide a superior expansion joint which is substantially more economical to manufacture than prior-art joints of comparable pressure ratings.
Another object of the invention is to provide an expansion joint of the flowing arch configuration but which can be formed in large diameter sizes with or without the use of a mold.
Still another object is to provide a joint of the low-arch type which can be made in larger sizes than heretofore practical.